CELL-SURFACE GLYCOCONJUGATES AND THE EXTRACELLULAR-MATRIX OF THE DEVELOPING MOUSE EMBRYO EPICARDIUM

Cell surface glycoconjugates and the extracellular matrix (ECM) of the proepicardium and the developing epicardium were studied in early mou se embryos by light and electron microscopy with histochaemical and im munocytochaemical techniques. The extracardially located proepicardium consists of polarized mesothelial cells forming the proepicardial ves icles. These vesicles contain a fine proteoglycan network and an acell ular ECM rich in hyaluronic acid. Membrane-bound glycoconjugates are s hown with cuprolinic blue, alcian blue and ruthenium red on the apical (outer) cell surface, while fibronectin and laminin are present on th e basal (luminal) cell surface. These membrane and matrix components o f the proepicardium might be involved in specific attachment of proepi cardial cells to the bare heart tube and might facilitate the initial migration of epicardial cells over the myocardial surface. In the cell coat of the cardiomyocytes of the bare heart tube the fibronectin and laminin are concentrated in patches. The formation of the epicardial covering is a rapid process, requiring only about 2 days (9-11 days) t o ensheath the entire heart tube from the inflow to the outflow segmen t. The subepicardial matrix between the newly formed epicardial coveri ng and myocardial layer is acellular at first, but contains a condensi ng proteoglycan network, membrane and matrix fibronectin, type IV coll agen and laminin on the myocardial cell surface. The formation and the distribution of the subepicardial ECM show regional characteristics. The accumulating ECM forms wide subepicardial spaces and protuberances in the atrioventricular and interventricular sulci. The sulci of the heart seem to provide the optimum microenvironment for haematopoiesis and vasculogenesis. Haematopoietic islands and coronary vessel forerun ners appear and concentrate in the regularly spaced surface protuberan ces. The vasculogenesis proceeds from the inflow to the outflow segmen t of the heart. The first blood capillaries appear in the sinoatrial s ulcus of the 10-day embryo. By 11-13 days the subepicardial blood vess els form an interconnected network and establish the coronary artery o rifices.